Technical Field
The present invention relates to the field of electrochemical sensors. Such sensors are utilized for analysis of, in particular, liquid process media in order to quantitatively determine specific properties of the process medium, in particular the pH value, the redox potential and/or the presence of certain ion species in the process medium. The properties that can be determined by means of electrochemical sensors will be summarized in the following under the term “electrochemical measurement variable”.
Description of the Related Art
For determining electrochemical measurement variables there are used suitable measuring electrodes having the property that, upon contact of the measuring electrode with the process medium, an electrochemical potential is created that is dependent upon the respective electrochemical variable to be determined, e.g., the pH value or the concentration of a specific ion species. This potential is sensed in the form of a measurement signal, so that such sensors are also referred to as potentiometric sensors. The measuring electrode may be in the form of an ion-selective electrode that establishes direct contact with the process medium to be investigated, so that the ion-selective electrode and the process medium directly constitute a redox system the electrochemical potential of which is dependent on the concentration of a specific ion species in the process medium. However, in many cases the measuring electrode itself will already comprise a system of a sensing electrode in contact with an inner electrolyte, with the inner electrolyte then establishing electrolytic contact with the process medium. For example, in case of pH electrodes, a potential difference dependent on the pH value of the process medium as a rule is measured via a glass membrane situated between the process medium and the inner electrolyte of the measuring electrode. The inner electrolyte then constitutes a buffer system and is selected such that an electrical potential difference as proportional as possible with respect to the desired electrochemical measurement variable is created across the measuring membrane. In quite general terms, a system comprising a measuring electrode and optionally an associated inner electrolyte/measuring membrane shall be referred to as measuring half-cell of an electrochemical sensor in the following, with the measuring electrode being provided in a measuring half-cell space containing also the inner electrolyte.
Due to the fact that electrical potentials always can be sensed merely as a difference from a reference potential, an electrochemical sensor always needs a measuring electrode and a further reference electrode that delivers an electrochemical reference potential as reference quantity for the measurement signal delivered from the measuring electrode. The reference electrode comprises a redox system consisting of a reference sensing electrode in contact with a reference electrolyte. The reference redox system is selected such that, upon contact of the reference sensing electrode with the reference electrolyte, a known electrochemical potential is created that is as stable as possible and permits the best possible reproduction. An arbitrary reference redox system of the type described will be referred to in quite general terms in the following as reference half-cell of an electrochemical sensor. The reference half-cell comprises a reference half-cell space containing the reference electrode and the reference electrolyte. The electrochemical potential of the reference half-cell is to be changed as little as possible by electrolytic contact of the reference electrolyte with the process medium.
Commonly used reference half-cells are based on electrodes of the second kind in which a reference sensing electrode of metal, which is coated with a not readily soluble metal salt, is in contact with a reference electrolyte containing a chemically inert salt that is readily soluble in the reference electrolyte and has the same anion as that of the not readily soluble electrolyte coating. The reference electrolyte frequently is a liquid or a rather viscous gel, in particular an aqueous saline solution or an aqueous gel of a specific salt. A redox system commonly used for reference electrodes, as it may also be provided for use with the present invention, consists, e.g., of an Ag electrode having a coating of AgCl and being immersed in a reference electrolyte of aqueous KCl. Other redox systems that are feasible for use with the present invention as well are based on iodine/iodide redox systems or calomel redox systems (Hg/Hg2Cl2), each together with suitable reference electrolytes selected on the basis of the criterion that the reference electrolyte should exhibit good electric conductivity, should be chemically neutral and the ions thereof should display as equal mobility as possible.
For determination of the respective electrochemical measurement variable, the reference half-cell has to be brought into electrolytic contact with the process medium to be investigated. It is then possible to determine a voltage difference corresponding to the difference of the electrochemical potentials between the reference half-cell and the measuring half-cell.
The respective electrochemical potential arising between measuring half-cell and process medium as well as between reference electrode and reference electrolyte is dependent on numerous factors. It is therefore necessary when putting an electrochemical sensor into operation to firstly perform a calibration with solutions or electrolytes of known ion activity (as a rule there are used standardized buffer solutions for this purpose) before the desired electrochemical measurement variable can be determined quantitatively from the voltage difference ascertained for an unknown process medium.
The electrochemical potential of the reference half-cell is sensitive with respect to environmental conditions, in particular when a reference electrode was in storage for a longer period of time. For permitting electrolytic contact of the reference electrode with the process medium, which is necessary for the measurement, the reference half-cell is in contact with its environment via an opening, a diaphragm or a similar connection. This entails the risk that the reference electrolyte either leaks or dries out in case of longer storage so that the reference electrode in part or completely is no longer in contact with reference electrolyte. This affects the equilibrium of the reference redox system so that complex measures are necessary when a sensor is put into operation before quantitative determinations are possible.
The present invention is concerned in particular with making available an improved reference electrode assembly for an electrochemical sensor of the type described. In addition thereto, an improved electrochemical sensor is to be indicated as well.
The document DE 10 2013 101 735 A1 shows a potentiometric sensor device comprising a reference electrode provided in a reference half-cell space and a measuring electrode provided in a measuring half-cell space, between which a potential difference is sensed upon electrolytic contact with a process medium. The measuring electrode comprises a measuring membrane dependent on the value of the variable to be analyzed, e.g., the pH value, of a process medium. The reference half-cell is filled with a largely dry or low-moisture substance (e.g., a dry electrolyte salt) which forms the reference electrolyte only by contact with a liquid. In this manner, at least in case of proper dry storage, there is not caused uncontrolled drying of the reference electrolyte. However, when the sensor is put into operation, the reference electrolyte has to be formed first by adding liquid, and a redox equilibrium with the reference electrode needs to be established.
In case of the potentiometric sensor shown in DE 10 2010 063 031 A1 the reference half-cell space, in the storage state, is closed off with respect to the environment by a wall and is filled with a reference electrolyte. This is to avoid that the reference electrolyte, during dry storage of the same, establishes contact with the environment and is subject to leakage, so that the reference half-cell space may possibly dry out. However, the sensor is not operable in its storage state, as the reference electrolyte cannot establish electrolytic contact with the process medium. For transferring the sensor into an operative state, there can be formed a through-opening in a wall confining the reference half-cell space, through which reference electrolyte can flow out from the reference half-cell space and thus establish electrolytic contact between the reference electrolyte and a process medium surrounding the reference half-cell space. As a means for providing the through-opening, there is provided a mechanical tool for penetrating the wall or for removing a plug or closure in the wall. The thus created through-opening as a rule can be controlled with difficulty only. Above all it is problematic that a very large opening is created in many cases through which the reference electrolyte is discharged from the reference half-cell space relatively rapidly and in uncontrolled manner. The period of operation of the sensor then may be very short and hard to predict. These problems arise in particular when liquid reference electrolytes, such as KCl, are used.